Although the P2 line does have a manual mode it was designed primarily as a camera platform and unless you are or have many hours of experience leave it be.
If you know how to fly Atti, manual mode only takes away the barometer. So you have to apply left stick up/down constantly to keep the same altitude. Don't make any harsh stick movements and you should be fine. You can always switch back if you don't trust it anymore. Make sure there's enough satellites, also in manual.
I used my P2 in manual to train for my commercial RPA Pilots Flight school exam. At the Netherlands Aerospace Laboratory, where I got my training, we were only allowed to fly in full manual and were not allowed to fly the training and exam with our own craft. I had to do the exam with a (by then still pretty much) unknown and rather big quad in full manual. During a code orange weather forecast, winds of 8m/s. Fortunately I have some 40 years of RC fixed wing and a couple of years helicopter experience. So I passed. Thanks to the phantom manual training sessions I did at home.
Don't do it. There is no trim setting on the P2's. This means if the aircraft is not perfectly balanced it will not respond accurately to your stick commands.
If you are attempting this, get some decent altitude as the Phantom will drop like a rock. And keep your finger on that switch as it can go pear shape real quick.
